There are a bewildering array of products for mixing your mediums and cleaning brushes, even when you’re only looking at one type of painting. Each oil has distinct properties, every paint manufacturer has their own trademarked medium, and everyone has a favorite combination they swear by. When it comes down to it, though, every blob of paint ever, from fine art tools to house paint, has three basic parts.
Three Things Make a Paint
- Pigment
The more the better! Pigment load is a huge part of the difference between artist-grade and ‘student quality’ paints. - Binding medium
The binder holds the pigment particles together both before and after drying. The type of binder defines the type of paint. Oil paints are made with drying oils (linseed, safflower, walnut), acrylics with polymer emulsion, watercolors with gum arabic, and egg tempera with actual factual egg out of a chicken. (Useful things, chickens). - Thinner / Dilutent
Either a solvent (mineral spirits, turpentine) for oils, or water for acrylics and watercolor.
In oil painting parlance, the oils are pure ‘fat’. Everything else is on a scale of diminishing fat: pre-mixed painting mediums and the paints themselves (mixtures of ‘fat’ oil and ‘lean’ pigment) are all somewhere in the middle, and vary based on manufacturer, product, and pigment. Solvent, having no oil at all, is entirely ‘lean’.
Because oils dry by capturing oxygen, not evaporating water, they expand as they dry. If the top layer dries more quickly than the layer below, it will have expanded as much as it can and hardened while the layer beneath is still moving, leading to a cracked painting. So ‘fat’ (slow drying) layers should be on top of ‘lean’ (fast drying) layers.
Oil manufacturer Gamblin has a great explanation of fat-over-lean painting.
Thankfully this is all a lot less delicate than it used to be, because modern alkyd (polymerized oil) painting mediums have some flexibility in their dry state. You don’t have to be exact with your mixed proportions, just don’t make huge changes in fat content from layer to layer and try to work in a generally fatter direction. If you finish your painting with glazes, they should be made with medium or medium and oil, not with solvent. This will also make the painting stronger, because solvent alone will leave your poor pigment stranded on the canvas with no friends to hang onto, while oil and oil medium bind the pigments into a paint film — the thing that makes it paint and not pretty colored dust.
Starting work: Two ways to do fat over lean
I covered this discarded painting with a cad yellow/titanium white mix thinned with 50/50 galkyd lite/OMS. The high ratio of solvent should help it dry faster (it’s very ‘lean’). It doesn’t matter how I painted the bit I’m covering over because it’s been sitting around the house for six years and is as dry as it can get. I’ll use less solvent and more galkyd as the painting progresses.
By contrast, I started this fiddle painting with paint straight from the tube. I used straight paint because I hadn’t used oils in over six years and I’d forgotten everything I ever knew (I don’t recommend this ‘learning’ technique, but it IS exciting!).
Thankfully, a medium/oil mix is fatter than straight paint, and pure oil fatter still, so I’ll add in some medium for the middle and oil/medium mix for glazes.
Of course, none of this fat-management matters if you like to paint alla prima (aka wet on wet or ‘all in one go’). Alla prima painting requires you to get everything right the first time, which is practically cheating. What a ridiculous notion.